Herbein et al. Retrovirology 2010, 7:34 http://www.retrovirology.com/content/7/1/34 Open Access REVIEW BioMed Central © 2010 Herbein et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Review Macrophage signaling in HIV-1 infection Georges Herbein* 1 , Gabriel Gras 2 , Kashif Aziz Khan 1 and Wasim Abbas 1 Abstract The human immunodeficiency virus-1 (HIV-1) is a member of the lentivirus genus. The virus does not rely exclusively on the host cell machinery, but also on viral proteins that act as molecular switches during the viral life cycle which play significant functions in viral pathogenesis, notably by modulating cell signaling. The role of HIV-1 proteins (Nef, Tat, Vpr, and gp120) in modulating macrophage signaling has been recently unveiled. Accessory, regulatory, and structural HIV- 1 proteins interact with signaling pathways in infected macrophages. In addition, exogenous Nef, Tat, Vpr, and gp120 proteins have been detected in the serum of HIV-1 infected patients. Possibly, these proteins are released by infected/ apoptotic cells. Exogenous accessory regulatory HIV-1 proteins are able to enter macrophages and modulate cellular machineries including those that affect viral transcription. Furthermore HIV-1 proteins, e.g., gp120, may exert their effects by interacting with cell surface membrane receptors, especially chemokine co-receptors. By activating the signaling pathways such as NF-kappaB, MAP kinase (MAPK) and JAK/STAT, HIV-1 proteins promote viral replication by stimulating transcription from the long terminal repeat (LTR) in infected macrophages; they are also involved in macrophage-mediated bystander T cell apoptosis. The role of HIV-1 proteins in the modulation of macrophage signaling will be discussed in regard to the formation of viral reservoirs and macrophage-mediated T cell apoptosis during HIV-1 infection. Introduction HIV-1 infection is characterized by sustained activation of the immune system. As macrophages, along with other cell types, are permissive to HIV-1 infection, they may be infected by the virus, resulting in signaling modulation [1]. Even uninfected macrophages may be activated by the soluble gp120 HIV-1 protein, or gp120 virion, via sev- eral signaling pathways. Additionally, soluble HIV-1 pro- teins such as Nef, Tat, and Vpr have been detected in serum of HIV-1 infected patients, possibly released by infected/apoptotic cells. Soluble exogenous HIV-1 pro- teins are able to enter macrophages and modulate both cellular machinery and viral transcription. Deciphering the signaling pathways involved in the activation of mac- rophages in HIV infection is critical to a better under- standing of AIDS pathogenesis as this could lead to innovative therapeutic approaches. HIV-1 Proteins and Macrophage Signaling Nef Nef is a 27-kDa myristylated protein which is expressed early in the virus life cycle. Nef down-regulates the cell surface expression of CD4, CD28, and MHC class I [2]. Nef also modulates several signaling pathways [3-8]. While Nef is not considered to be a secreted protein, exogenous Nef has been detected in the sera of AIDS patients and in cultures of HIV-1-infected cells [9]. There is increasing evidence of the ability of extracellular Nef to activate signaling pathways in uninfected cells [9-13]. Indeed, Nef is internalized by MDMs and dendritic cells, but not by T cells [14], when added to cell cultures [14- 16]. Recently, Qiao et al. [11] reported that Nef was inter- nalized in B cells in vitro, thereby suppressing CD40- dependent immunoglobulin class switching. The pres- ence of Nef in the sera of HIV-infected patients at con- centrations ranging from 1 to 10 ng/mL has also been described [9]. This concentration may be higher in the lymphonodal germinal centers where virion-trapping dendritic cells, as well as virion-infected CD4+ T cells and macrophages, are densely packed [17,18]. Infected cells may release Nef through a non-classical secretory pathway or after lysis. Following this, bystander cells may internalize Nef via endocytosis, pinocytosis or other yet- * Correspondence: georges.herbein@univ-fcomte.fr 1 Department of Virology, UPRES 4266 Pathogens and Inflammation, IFR 133 INSERM, University of Franche-Comté, CHU Besançon, F-25030 Besançon, France Full list of author information is available at the end of the article Herbein et al. Retrovirology 2010, 7:34 http://www.retrovirology.com/content/7/1/34 Page 2 of 13 unknown mechanisms. Regarding intracellular signaling induced by Nef treatment of MDMs, it has been reported that Nef modulates the expression of a significant num- ber of genes as early as 2 hours after treatment [19]. This suggested that a prompt transcriptional cell reprogram- ming induced by Nef leads to the synthesis and the release of pro-inflammatory cytokines/chemokines, which in turn, activate STAT1 and STAT3 signal trans- ducers and transcription activators [20,21]. In line with these results, Nef treatment of MDMs was reported to induce rapid activation of IKK/NF-kB, MAPK and IRF-3 signalling pathways. Nef induces prompt phosphoryla- tion of three MAPKs, i.e., ERK1/2, JNK, and p38 [13,22,23]. A Nef treatment as short as 15 minutes is able to induce p38 phosphorylation, most likely due to rapid recruitment and activation of p38 signaling upstream intermediates. Exogenously added Nef induces rapid phosphorylation of the transcription factor IRF-3, the main regulator of IFN-β gene expression [24-26]. It has also been shown to induce tyrosine phosphorylation of STAT2, well known to be induced by type I IFN signaling, at an early infection stage (8 to 16 h) [22]. Macrophage activation and production of pro-inflam- matory cytokines by Nef involves NF-κB activation, espe- cially its p50/p50 homodimeric and p65/p50 heterodimeric forms. This event leads to sustained LTR activation [13,19,27]. The activation of NF-κB in mac- rophages treated with exogenously added Nef occurs as early as 2 hours after treatment [13,28]. NF-κB activation in primary macrophages treated with recombinant Nef is mediated via the canonical pathway, primarily involving IKKβ phosphorylation [28]. Furthermore, many of the transcripts induced in macrophages treated by Nef are encoded by genes regulated by κB-like responsive ele- ments [19] (Figure 1). Therefore, there is evidence that exogenously added Nef plays a critical role in "hijacking" the NF-κB signaling pathway, most likely upstream of IKK, as observed after endogenous expression in mac- rophages [29]. This observation is in line with the role of Nef-mediated activation of NF-κB, which promotes HIV- 1 replication via both direct and cytokine-mediated effects [13]. Thus, in monocyte-derived macrophages, recombinant Nef enhances the production of cytokines such as macrophage inflammatory protein-1 alpha (MIP1α), MIP1β, TNFα, IL-1β and IL-6 involved in the inflammatory response (Figure 1). Additionally, features observed in promonocytic cells and primary mac- rophages following exposure to recombinant Nef are very similar to those observed following TNFα treatment [30]. Both recombinant Nef and TNFα activate NF-κB, AP-1 and JNK. That recombinant Nef and TNFα activate these signaling pathways suggests the two events might modu- late the cellular machinery in a similar way. Therefore, they may have the same effects on HIV-1 replication in mononuclear phagocytes [28]. Exogenous Nef may mod- ulate intracellular signaling pathways downstream of the TNFα receptors (TNFRs), and thus mimic the effects of TNFα on primary macrophages [13]. Tat HIV-1 Tat is a virally encoded transactivating protein which plays a critical role in viral replication and is con- served in genomes of primate lentiviruses [31,32]. Tat is a HIV-1 protein reportedly detected in the sera of infected Figure 1 HIV-1 proteins modulate signaling in the macrophage. The HIV-1 proteins Nef, Tat, Vpr, and gp120 alter cell signaling path- ways, both in infected and uninfected macrophages. The presence of exogenous Nef, Tat and Vpr has been reported in sera of AIDS patients, which have the ability to enter the cells. HIV-1 proteins activate multi- ple transcription factors in macrophages including NF-κB, Sp-1 and AP- 1, which have binding sites in the long terminal repeat (LTR) of HIV-1. The induction of these factors results in increased viral production. Fur- thermore, the activation of these transcription factors enhances cy- tokine production by macrophages primarily involved in AIDS pathogenesis. TNF promoter is shown as a prototype containing bind- ing sites of NF-κB, Sp-1, and AP-1. Exogenous Nef and Vpr may en- hance Tat-mediated transcription in addition to their effect on transcription factors. Moreover, the viral glycoprotein gp120 activates MAPK in uninfected and infected cells, resulting in increased TNFα pro- duction through ATF-2 binding sites of its promoter. Tat also stimu- lates CXCR4/CCR5 surface co-receptor expression, thus enhancing viral entry in cells. Besides LTR activation through transcription factors, Vpr-induced cell cycle arrest facilitates LTR stimulation. UNINFECTED Mĭ CXCR4 CCR5 Cell cycle arrest + + INFECTED Mĭ + Ĺ Viral Transcription HIV-1 LTR Tat MAPK NF- N B, AP-1, Sp-1 Vpr Nef Lyn PI3K gp120 TNF promoter MAPK CXCR4 CCR5 Ĺ Production of TNF gp120 Lyn PI3K CXCR4 CCR5 ATF-2 ATF-2 AP-1 AP-1 NF-țB NF-țB TAR Sp-1 Sp-1 AP-1 NF-țB NF-țB ATF-2 Sp-1 Sp-1 Sp-1 Ĺ Production of TNF TNF promoter Vpr Vpr Tat Tat Nef Nef + HIV-1 infection HIV-1 infection AP-1 NF-țB NF-țB ATF-2 Sp-1 Sp-1 Sp-1 CXCR4 CCR5 NF- N B, AP-1, Sp-1 Herbein et al. Retrovirology 2010, 7:34 http://www.retrovirology.com/content/7/1/34 Page 3 of 13 patients as well as in the media of infected cells [33]. This suggests that it might have a role both as endogenous modulator of cellular functions within infected cells and act on bystander cells. Tat activates monocytes, mac- rophages, and microglial cells. Tat Action on monocytes, macrophages, and monocytic cell lines The HIV-1 Tat protein is essential for efficient transcrip- tion of viral genes and for viral replication. It also regu- lates the expression of several cellular genes and interferes with intracellular signaling [34,35]. The mature protein has a variable size, ranging from 86 to 101 amino acids. It is organized in functional domains required for transactivation activity. The C-terminus contains an RDG motif which mediates cell adhesion and Tat binding to integrin receptors [36]. Specific Tat binding has been reported for at least three cell surface molecules includ- ing heparin sulfate, beta-integrin and chemokine recep- tors. Tat as well as peptides spanning its cysteine-rich region compete with cognate ligands to bind CXCR4, CCR2, and CCR3 chemokine receptors in primary human monocytes and PBMCs. Tat has also been reported to trigger Ca 2+ mobilization in macrophages in a concentration-dependent manner through CCR2 and CCR3 [37,38]. Moreover, Tat induces the expression of CCR3, CCR5 and CXCR4 in monocytes/macrophages in a concentration-dependent manner, possibly promoting HIV-1 infection [39]. Finally, Tat has been shown to serve as chemoattractant for monocytes, and pretreatment with Tat enhanced the monocyte invasive properties [40,41]. Functional consequences of Tat activation include TNFα release from macrophages, monocytes and THP-1 monocytic cell lines [42]. Tat-induced TNFα release was dependent on NF-κB activation and mediated through the activation of protein kinase A, phospholipase C (PLC) and protein tyrosine kinase pathways [43]. Transient [Ca 2+ ]i release was observed in macrophages through IP3 receptor-regulated intracellular Ca 2+ stores [43]. This Tat- induced [Ca 2+ ]i elevation was not dependent on extracel- lular Ca 2+ or caffeine-sensitive ryanodine receptor-regu- lated intracellular Ca 2+ stores but rather on the PLC, protein kinase C (PKC) and Gi/0 protein pathways. Tat- induced calcium signaling in macrophages leads to the production of pro-inflammatory cytokines and chemok- ines, possibly contributing to inflammation and HIV-1 neuropathogenesis. Thus, Tat displays biological activities mimicking those mediated by TNFα [28]. HIV-1 Tat may induce the expression of TNFα and various cytokines, including IL- 6, TNFβ and TGFβ as well as the expression of cytokine receptors such as the IL-4 receptor [44-49]. Like TNFα, Tat may activate NF-κB, AP-1 and MAPK, including c- Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) [50]. Tat activates NF-κB, JNK, and AP-1, but not MEK [50]. These results suggest that HIV-1 Tat and TNFα act through different mechanisms and that HIV-1 Tat does not activate all of the kinases involved in TNFR signaling [51]. In short, like Nef, Tat mimics the effects of TNFα resulting in the enhancement of viral rep- lication via activation of NF-κB, AP-1, JNK, and MAPK. Action of Tat on microglia Tat protein is actively produced and released in the cen- tral nervous system (CNS) by infected cells [52]. Elevated Tat mRNA levels have been detected in the brain of AIDS patients [53], where Tat is believed to play a significant role in the pathogenesis of HAD through not only its direct neurotoxicity, but also through the release of dele- terious products in microglial cells [54]. Although they act as CNS macrophages, microglia cells differ in many aspects from peripheral macrophages. Their morphologi- cal and functional specificity responds to cell-cell con- tacts and secreted factors from surrounding astrocytes and neurons. The strict separation of microglia cells from blood components is due to the blood brain barrier (BBB). This results in a down-regulated "surveillance" phenotype [55]. Microglial cells are nevertheless able to undergo activation and acquire typical macrophage func- tions such as phagocytosis of microbes or apoptotic bod- ies and the secretion of inflammatory or anti- inflammatory mediators [56,57]. Tat activates microglia and impairs major molecular mechanisms that normally prevent or shorten microglial activation. As is the case in macrophages, Tat increases microglial production of free radicals as well as pro- inflammatory cytokines and chemokines [42,43,58,59]. Tat induction of NO and inducible NO synthase (iNOS) is enhanced by IFN-γ [60]. This suggests that Tat and IFN-γ cooperatively contribute to the severity of brain damage observed in brain tissues from AIDS patients and animal HAD models. The transcription factor NF-κB plays a central role in the regulation of inflammatory gene expression and is involved in most Tat-induced effects in microglial cul- tures [61]. In surveillance microglia, signals provided by astrocytes actively contribute to NF-κB down-modula- tion [62]. Elevated immunoreactivity for p50/p65 het- erodimer subunits was found in microglia and brain macrophages of children with HIV encephalitis [63] despite repression by the surrounding cells. Likewise, nuclear staining for NF-κB in the perivascular microglia/ macrophages of deep white matter and basal ganglia cor- related with the severity of HIV-associated dementia in AIDS patients [64]. Interestingly, Tat-induced formation of free radicals in microglial cells occurs independently from NF-κB activation [65,66], as lipid peroxidation and oxidative stress still occur in microglial cultures exposed to Tat in the presence of NF-κB inhibitors [65]. Likewise, Herbein et al. Retrovirology 2010, 7:34 http://www.retrovirology.com/content/7/1/34 Page 4 of 13 the pro-oxidant activities of Tat in the N9 microglial cell line depend on MAP kinase activation [66]. Additionally, antioxidants abrogate oxidative stress rather than the other Tat-induced functions such as IL-1β, NO, and TNF-α production or IkBα degradation [65]. Thus, Tat- induced NF-κB activation in microglia may not require the formation of free radicals, although oxidative stress is contributive to its activation [67]. In different cell types, including macrophages and microglia, Tat influences cell function by modifying Ca 2+ homeostasis [43]. Indeed, Tat possesses a cysteine- cysteine-phenylalanine domain, enabling Tat to mimic beta chemokine effects on both Ca 2+ movements and chemotaxis [38]. In microglia, Ca 2+ mobilization and cell migration by Tat are sensitive to pertussis toxin (PTX), but not cholera toxin. This observation supports the involvement of Gi rather than Gs type proteins, as expected for chemokine receptor stimulation [37]. Fur- thermore, cross-desensitization studies revealed CCR3 receptor involvement. Similar to findings in monocytes, Tat-induced Ca 2+ signals in human microglia are charac- terized by rapid desensitization [68]. Nanomolar concentrations of recombinant Tat have been shown to decrease in a dose- and time-dependent manner, cAMP accumulation induced in microglial cul- tures by the β-adrenergic receptor agonist isoproterenol, or by forskolin, an activator of adenylyl cyclase [69]. In microglia, increased cAMP accumulation lowers poten- tially neurotoxic pro-inflammatory molecules [70-76] and promotes the production of neuroprotective or immunosuppressive substances [70]. Thus, Tat may inter- fere with cAMP's control on microglial activation. Among the ion channels expressed by microglial cells, there are two major classes of K+-permeable channels: the delayed-outward-rectifying (Kdr) and the inward-rec- tifying (Kir) channels. Their expression differs in mac- rophages and microglia. Their expression is finely modulated by both activation and differentiation [77-81]. Chronic microglial cell treatment with high Tat concen- tration (≥ 100 ng/mL) up-regulates Kdr currents due to NF-κB-dependent increase in channel expression without a significant increase in Kdr currents [82]. Therefore, the hyperpolarization thus induced by Tat may has several consequences. Ca 2+ influx depends on a hyperpolarized membrane potential and Tat's β-chemokine mimicry may thus be favored by Kdr currents. Kdr currents may also modulate the microglial respiratory burst and the trans- port of amino acids through voltage-dependent trans- porters. The latter is likely to modify the availability of amino acids for protein synthesis [83,84], as well as the dynamics of glutamate exchange between intracellular and extracellular pools. This may affect the regulation of both extracellular glutamate concentration in the vicinity of glutamate-sensitive neurons and glutathione synthesis rate in microglia [85-87]. Vpr Vpr is a 96 amino acid-long virion-associated protein located in the cytoplasm and nucleus of HIV-infected cells [88-93]. Vpr is not essential for viral replication in T cells, but critical for HIV replication in non-dividing cells such as macrophages [94-99]. Vpr has pleiotropic effects on viral replication, cellular proliferation and differentia- tion, cytokine production, NF-κB-mediated transcription and apoptosis [100-103]. Vpr has been shown to induce cell cycle arrest at the G2 cell cycle phase [104-107]. G2 cell cycle arrest correlates with the inhibition of Cdc2 activity and parallels enhanced viral replication [108-110]. G2 cell cycle arrest is followed by apoptosis in HIV-infected and Vpr- expressing cells [111]. Apoptosis is mediated through the interaction of Vpr with the mitochondrion permeability transition pore. This interaction opens the pore, causing mitochondrial swelling, release of cytochrome C as well as caspase 9 and caspase 3 activation [111]. p53 tumor suppressor protein may be implicated in cell cycle arrest and apoptosis mediated by Vpr in certain cell types [107]. Vpr transactivates the viral promoter and HIV-1 LTR resulting in increased viral replication. The G2 cell cycle arrest is concomitant with high levels of viral replication in primary human CD4+ T cells. An interaction between Vpr, Sp1 and TFIIB transcription factors is required for Vpr-mediated transcriptional enhancement of HIV-1 LTR [112,113]. Vpr-mediated transactivation necessitates intact NF-κB sites and depends on Vpr's ability to stimulate p300/CBP coactivator function, which promotes cooperative inter- action between the RelA subunit of NF-κB and the cyclin B1Cdc2 [114]. A structural and functional interaction between Vpr and Tat has been reported, synergistically enhancing the transcriptional activity of the HIV-1 LTR [114]. The activity of recombinant Vpr (rVpr) in macrophages has been investigated. High concentrations of rVpr as well as the carboxy-terminal Vpr peptide are cytotoxic to macrophages. However, at low concentrations rVpr was shown to enhance the activity of several transcription factors including AP-1, c-Jun, and, NF-κB [115]. Amino- and carboxy-terminal Vpr peptides retained transcription factor activation properties, albeit to a lesser extent than with the full-length rVpr. Similarly to Vpr expressed in infected cells, rVpr stimulated HIV-1 replication in acutely infected primary macrophages. Furthermore, reduced p24 production by macrophages infected with Vpr-deficient virus could be rescued by adding rVpr to culture medium [116]. Exposure to rVpr also increased transcription and p21/waf1 levels in macrophages [117]. Herbein et al. Retrovirology 2010, 7:34 http://www.retrovirology.com/content/7/1/34 Page 5 of 13 These Vpr effects on macrophages may reflect the mech- anisms by which Vpr activates the HIV-1 LTR and enhances virus replication in acutely and latently infected cells [88]. Although primarily considered to be a regula- tor of viral promoter transactivation, transcription factor activation may have significant effects on macrophage cellular functions [117,118]. Additionally, macrophages and PBLs produce less chemokines following recombi- nant Vpr treatment. This observation suggests that Vpr modulates cytokine production by interfering with NF- κB-mediated transcription [119,120]. gp120 HIV-1 infects human T cells and monocytes/mac- rophages through the interaction of gp120 with CD4 and the CXCR4 or CCR5 co-receptor, which determines the cellular tropism [121-131]. HIV-1 gp120 down-regulates CD4 expression in primary human macrophages through induction of endogenous TNFα [121,132-136]. Actually, TNFα down-regulates both surface and total CD4 expres- sion in primary human macrophages at the transcription level [134,137-140]. TNFα inhibits R5 and R5/X4 HIV-1 entry into primary macrophages via downregulation of both cell surface CD4 and CCR5 and via enhanced secre- tion of CC-chemokines, MIP-1α, MIP-1β and RANTES [129,137,141-146]. An iterative pretreatment of primary macrophages with TNFα prior to HIV infection inhibits HIV-1 replication in primary macrophages [142]. The inhibition of HIV-1 entry into primary macrophages fol- lowing TNFα pretreatment involves TNFR2 and is medi- ated by the secretion of CC-chemokines such as RANTES, MIP-1α, and MIP-1β[140,141]. TNFα induces the production of RANTES, MIP-1α, and MIP-1β, which in turn down-regulate cell surface CCR5 expression on primary macrophages, resulting in the inhibition of R5 HIV-1 entry [147-151]. In agreement with this observa- tion RANTES inhibits HIV-1 envelope-mediated mem- brane fusion in primary macrophages [152] and inhibits the activity of the RANTES promoter containing four NF-κB binding sites which is up-regulated by TNFα [153]. Many studies conducted over the past two decades have shown that besides infection, exposure of mac- rophages to intact virions or soluble gp120 may exert var- ious functional effects on macrophages, including cytokine secretion activation [121,135,154]. However, the specific pathways involved in gp120-induced responses have only been defined recently. The presence of non- infectious virion particles in excess of infectious virus, the ability of gp120 to dissociate from the transmembrane gp41 portion of Env as well as detection of circulating gp120 in infected patients [155] have raised the question of what biological activities this protein is involved in aside from mediating infection. Such studies have dem- onstrated the ability of gp120 to activate intracellular sig- naling in multiple cell types as a result of its binding to receptor/co-receptor complex. Although gp120-induced signaling has been extensively investigated in CD4+ T cells, gp120 has also been reported to activate intracellu- lar signals in macrophages [156]. In primary human macrophages, both R5 and X4 gp120 induce calcium mobilization, although R5 gp120 elicited higher peaks and more sustained elevations than X4 gp120 [157,158]. Single-cell patch-clamp recording com- bined with pharmacological antagonists and current reversal potential analysis identified the ion channels associated with CCR5 and CXCR4 activation: chloride, calcium-activated potassium, and non-selective cation (NSC) channels [157]. These responses to HIV-1 gp120 were mediated by chemokine receptors, but not by CD4, since the responses to R5 Env were absent in mac- rophages from patients lacking cell surface CCR5 expres- sion (CCR5Δ 32); responses to X4 gp120 were inhibited by a small molecule CXCR4 antagonist [157,159]. While R5 and X4 gp120 generally induced similar signals through CCR5 and CXCR4, respectively, certain differ- ences were noted. R5 Env opened the calcium-activated outward K + channels more frequently than X4 gp120, and induced Cl- currents of greater amplitude. Gp120, instead of CXCR4 or CCR5 binding chemokines, acti- vated the NSC channel [160]. In addition, gp120 has been shown to activate all three MAPK family members (ERK1/2, JNK, and p38) in mac- rophages. R5 gp120 triggered macrophage release of MIP-1, MCP-1, and TNFα. The secretion of these prod- ucts was blocked by small molecule inhibitors of ERK1/2 and p38 MAPKs [39,161]. The src kinases Lyn and Hck are highly expressed in macrophages, and recent in vitro kinase assays demon- strated that R5 gp120 and MIP-1β activated Lyn in mac- rophages [162]. Neither R5 gp120 nor MIP-1β activated Lyn in macrophages derived from CCR5Δ 32 donors or in cells treated with a small molecule CCR5 inhibitor, indi- cating that Lyn activation was elicited through CCR5 receptor. Unlike Lyn, Hck activation did not occur in response to gp120 or chemokine stimulation [162,163]. Both a Lyn-specific peptide pseudo-substrate inhibitor and PP2, a broad src family kinase inhibitor, suppressed gp120-induced TNFα production. These results are sug- gestive of a signaling cascade initiated by gp120 through CCR5, involving Lyn activation of the MAPK pathway, resulting in gp120-induced TNFα release. Several lines of evidence indicate that HIV-1 gp120/ chemokine receptor interactions activate PI3K in mac- rophages [39,164]. This finding is based upon R5 gp120 activation of protein kinase B (PKB), a downstream target for class I PI3K and a useful indirect indicator of its acti- vation. Furthermore, several small molecule PI3K inhibi- Herbein et al. Retrovirology 2010, 7:34 http://www.retrovirology.com/content/7/1/34 Page 6 of 13 tors blocked gp120-induced CCR5-mediated ERK1/2 and p38 phosphorylation, as well as TNFα release. These results not only suggest a role for PI3Ks in CCR5 signal- ing but also indicate that, like Lyn, PI3K acts upstream of MAPKs in the regulation of cytokine production through this pathway [39]. It is unclear which PI3K isoform is involved in these R5 gp120-induced signals, and the rela- tionship between PI3K and Lyn remains to be deter- mined. Besides chemokine receptors, interactions between HIV-1 gp120 and CD4 stimulate signal transduction pathways, such as activation of PKC, generation of PKC- dependent phosphorylation of CD4, and activation of the ERK/MAPK pathway, which in turn stimulates transcrip- tion factors such as NF-kB, AP-1, and Elk-1, as well as induction of cytokine and chemokine gene expression [115,165-172]. Early inflammatory gene products such as TNFα, may stimulate HIV-1 replication in the absence of HIV-1 Tat protein. Thus, the activation of cellular signal- ing pathways leading to the production of cytokine and chemokine genes by HIV-1 gp120 could facilitate viral replication in the early phases of the viral life cycle [50]. Proline-rich tyrosine kinase 2 (Pyk2) activation has been suggested as a critical signalling mechanism for integrin-mediated formation of adhesion contacts in macrophages known as podosomes. Pyk2 is known to be activated by chemokines, triggering cell migration [173,174]. CCR5 and CXCR4 are both linked to Pyk2, which is activated by R5 gp120 and MIP-1β as well as X4 gp120 and SDF-1α [161]. Recently, a functional role for Pyk2 in the migration of macrophages has been demon- strated using Pyk2 knockout mice [175], suggesting that gp120 may be involved in macrophage migration. Macrophage Signaling and HIV-1 Pathogenesis In this section, we report that several HIV-1 proteins may modulate the macrophage signaling pathway resulting in T lymphocytes depletion and viral cellular reservoir for- mation, especially in macrophages [176]. Macrophage signaling and T cell apoptosis Increased spontaneous and activation-induced apoptosis of peripheral CD4+ T cells from HIV-infected patients is observed ex vivo in lymph nodes of HIV-infected patients and of SIV-infected macaques [177-180]. Deciphering the molecular mechanisms involved in CD4+ T cell apoptosis in HIV-infected patients is critical to understanding HIV pathogenesis. In macrophages, Nef has been shown to activate multi- ple cellular pathways, possibly leading to increased infec- tion of adjacent T cells through bystander mechanisms involving T cell activation (Figure 2). It has been shown that Nef-expressing macrophages enhance resting CD4+ T cell permissiveness through a complex cellular and sol- uble interaction involving macrophages, B cells, and CD4+ T cells [29]. Nef expression within macrophages via adenoviral vectors has been shown to induce the secretion of soluble CD23 and ICAM, resulting in up-reg- ulation of costimulatory B cell receptors, including CD22, CD54, CD58, and CD80. This leads to T cell activation upon interaction with B cells via these costimulatory receptors, thus enabling the generation of non-produc- tive or productive reservoirs, depending on the interac- tions [29]. Furthermore, Nef has been reported to prevent Fas- and TNF-receptor-mediated deaths observed in HIV- infected T cells via interaction with the apoptosis signal regulating kinase-1 (ASK-1). Nef inhibits ASK-1, caspase 3 and caspase 8 activation, resulting in apoptosis block- ade in HIV-infected cells [181-184]. Apoptosis was mea- sured in productively infected CD4+ T lymphocytes using a reporter virus and a recombinant HIV infectious clone expressing the green fluorescent protein (GFP) in the presence and absence of autologous macrophages. The survival of productively infected CD4+ T lympho- cytes has been shown to require Nef expression and acti- Figure 2 A model of HIV-1 pathogenesis based on interactions between macrophages and T cells which account for increased immune suppression and cellular virion reservoirs. a) Viral glyco- protein gp120 activates the production of pro-inflammatory cytokines and chemokines by macrophages, attracting T cells in the vicinity of macrophages, thereby increasing the number of infected cells and fu- eling the viral reservoirs. HIV-1 proteins Nef, Tat, and Vpr activate the long terminal repeat (LTR) of HIV-1, resulting in sustained viral growth while also activating anti-apoptotic pathways that favor viral persis- tence and formation of viral reservoir. b) Viral protein Tat participates in CD4+ T cell death through TRAIL secretion by HIV-1 infected mac- rophages. Viral gp120 glycoproteins increase the expression of TNF and TNFR on macrophages and T cells, leading to CD8+ T cell apopto- sis. Thus, macrophage signaling using viral proteins accounts for both viral persistence and immune suppression during HIV-1 infection. Tat Nef Vpr CD4 + T cell as viral reservoir Inhibition of apoptosis Sustained viral transcription Ļ ASK1 Mĭ T8 T4 T4 Ĺ Expression of TNF and TNFR Ĺ production of proinflammatory cytokines & chemokines T4 T8 T4 T4 T8 T4 DR5 FAS CD8+ T cell Apoptosis TNF CD4+ T cell Apoptosis TRAIL FAS L Inflammatory cytokines Chemokines T4 T8 Apoptosis CD23, ICAM T cell activation T4 Inhibition of apoptosis Sustained viral transcription Macrophage as viral reservoir Recruitment of T cells T8 T4 T4 T8 T4 T4 Ĺ T cell infection T4 T4 T4 T4 T8 T cell activation Ĺ T cell infection Ļ Bcl-XL TNFR2 Apoptosis T4 b. Immune suppression a. Viral reservoir T8 T4 T4 T8 T4 Vpr Nef Tat gp120 Herbein et al. Retrovirology 2010, 7:34 http://www.retrovirology.com/content/7/1/34 Page 7 of 13 vation by TNFα expressed on macrophage surface, thereby participating in the formation and maintenance of viral reservoirs in HIV-infected patients [184]. In addition to the macrophage-mediated formation of T cell reservoirs, in vitro culture models demonstrate that uninfected CD4+ T cells undergo apoptosis upon contact with HIV-infected cells; for example mononuclear phago- cytes [180]. Macrophages play a major role in this pro- cess, suggesting that apoptosis-inducing ligands expressed by macrophages mediate apoptosis of suscepti- ble CD4+ T cells [159,185-187]. Activated macrophages produce TNFα following HIV infection in vitro [135]. TNFα is released as a soluble factor or expressed on the surface of macrophages under a membrane-bound form that primarily targets TNFR2 rather than TNFR1 [188,189]. TNFR2 stimulation may trigger T cell apopto- sis, especially in CD8+ T cells [188]. TNFα and TNF receptors are increased in HIV-infected patients and inversely correlated with CD4+ T cell counts [190]. TNFα is expressed on the surface of activated macrophages, and cell surface TNFR2 is not increased on CD4+ infected T cells. Therefore, for the most part, the apoptosis of CD4+ T lymphocytes is mediated via Fas/Fas ligand interaction [185,186,191]. TNFα causes death at a later stage than Fas and may be transduced through TNFR2, which does not contain homology to the Fas death domain and uses dif- ferent signaling pathways than TNFR1 [115,185]. Recently, Tat has been reported to induce secretion of soluble TNF-related apoptosis-induced ligand (TRAIL) in human macrophages, leading to the death of bystander CD4+ T lymphocytes [73]. Thus, the production of TRAIL by Tat-stimulated monocytes/macrophages is likely to be an additional mechanism by which HIV-1 infection destroys uninfected bystander cells. CD8+ T cell apoptosis during HIV infection has been shown to result from the interaction between membrane- bound TNFα expressed on the surface of activated mac- rophages and TNFR2 expressed on the surface of acti- vated CD8+ T cells [158]. Both membrane-bound TNFα and TNFR2 are up-regulated on macrophages and CD8+ T cells, respectively, following CXCR4 stimulation by HIV gp120. However, CCR5 may also play a role, albeit minor [158]. TNFR2 stimulation of T cells results in decreased intracellular levels of apoptosis protective pro- tein Bcl-XL, a member of the Bcl-2 family [192]. Impaired induction of Bcl-XL has been observed in PBMC isolated from HIV-infected patients [193]. Therefore, TNFR2 stimulation of CD8+ T cells by membrane-bound TNFα expressed on the surface of macrophages might decrease the intracellular levels of anti-apoptotic proteins resulting in CD8+ T cell death. Additionally, chemokines and activated macrophages have been reported to play a role in HIV-1 gp120-induced neuronal apoptosis [194,195]. Macrophage signaling and formation of viral reservoirs Whereas CD4+ T cells die within a few days after becom- ing infected with HIV, infected macrophages seem to per- sist for months, continuing to release viruses. Several reasons may explain why macrophages are a major cellu- lar reservoir of virions during infection (Figure 2). Mac- rophages are more resistant than T cells to HIV-induced apoptosis and therefore allow for sustained viral produc- tion without fatal cell death. Persistent HIV infection of macrophages results in increased NF-κB levels, involved in the resistance to TNFα-induced apoptosis. Mac- rophages release CC-chemokines which have the ability to attract CD4+ and CD8+ T lymphocytes in their vicin- ity [196]. They may also block the entry of R5 HIV-1 viri- ons into CD4+ target cells [122]. CC-chemokine production is often associated with that of pro-inflamma- tory cytokines, such as TNFα and IL-1β, which stimulate the transcription of HIV LTR via activation of NF-kB [197,198]. Additionally, TNFα may block entry of R5 HIV-1 strains into macrophages via a decreased expres- sion of CCR5 on cell surfaces [137,141,142,147]. Thus, CC-chemokines and pro-inflammatory cytokines facili- tate the recruitment and productive infection of CD4+ T lymphocytes via increased viral transcription, while regu- lating the entry of virions into macrophages, thereby pre- venting macrophage superinfection. Additionally, apoptosis inhibition in HIV-1 infected T cells enhances virus production and facilitates persistent infection [199]. HIV-1 proteins, by modulation of the TNFR signaling pathway, lead to the formation of viral reservoirs, espe- cially in primary macrophages [50]. Altogether, the data indicate that both viral and cellular factors are involved in the controlled and sustained production of virions in infected CD4+ T lymphocytes and macrophages, thereby expanding the viral reservoir which fuels disease progres- sion. Conclusion The macrophage is essential in the loss of T lymphocytes and formation of viral reservoirs; it plays a critical role in HIV-1 disease progression. Several HIV-1 proteins mod- ulate signaling in infected and bystander macrophages, thereby facilitating disease progression. A better under- standing of the manner by which HIV-1 modulates sig- naling in macrophages may be instrumental in the development of new therapeutic approaches that may ultimately restrict or decrease the size of cellular virion reservoirs in HIV-1-infected patients. Competing interests The authors declare that they have no competing interests. Authors' contributions GH was responsible for drafting and revising the manuscript as well as organiz- ing the content. GG was responsible for drafting and revising the section Herbein et al. Retrovirology 2010, 7:34 http://www.retrovirology.com/content/7/1/34 Page 8 of 13 "Action of Tat on microglia". KAK created Figures 1 and 2. WA assisted in revising the manuscript. Acknowledgements This review is the result of a reflection conducted by the Association for Mac- rophages and Infection Research (AMIR). The work of G. Herbein, W. Abbas, and K.A. Khan was supported by institutional funding from Franche-Comté Univer- sity. W. Abbas and K.A. Khan are supported by grants of the Higher Education Committee of Pakistan. The group led by G. Gras was supported by grants from the Agence nationale de recherche sur le sida et les hépatites virales, the Fondation pour la recherche médicale and Ensemble contre le sida (Sidaction). 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Even uninfected macrophages may be activated by the soluble gp120 HIV-1 protein, or gp120 virion, via sev- eral signaling pathways. Additionally, soluble HIV-1. is a HIV-1 protein reportedly detected in the sera of infected Figure 1 HIV-1 proteins modulate signaling in the macrophage. The HIV-1 proteins Nef, Tat, Vpr, and gp120 alter cell signaling path- ways,